It’s reasonably common for an HTTP 400 error to occur when accessing an Office 365 page. The Office 365 Admin Center seems to be particularly susceptible to the problem. What can you do when an Office 365 page barfs?

It's just a horrible problem that reflects very badly on Office 365. Once again the topic of testing comes to mind. If code is tested properly we shouldn't see issues like this in production... And running the Admin Center to see if a page paints properly isn't testing. The code has to be used to do real work and if it was, then surely bugs like this would show up. After all, even Microsoft engineers must become tired at having to blow away all the cached cookies just to access a web site...